This invention relates to electric power assisted steering apparatus of the kind in which an electric motor is adapted to apply an assistance torque to a steering component such as a steering column so as to reduce the driver effort required to control the vehicle.
In a simple electric power assisted steering (EPAS) apparatus a torque sensor is provided which is arranged so that the level of torque in a steering column is measured. From this measurement a controller calculates the value of a torque demand signal, which includes an assistance torque component that is indicative of the torque that is to be generated by an electric motor attached to the steering column. The motor applies an assistance torque to the column of the same sense as that demanded by the driver and thus reduces the effort needed to turn the wheel.
When such an apparatus is initially powered up, it may be the case that the controller calculates that a high assistance torque is required. Herein, “powered-up” and associated phrases should be read to include any state in which the EPAS apparatus is initialised after a loss of power. However, if such a torque were to be applied to the steering system immediately, then the resultant sudden movement of the steering system may alarm the driver of the vehicle. Accordingly, in prior art EPAS apparatus, the assistance torque limited to a value that is ramped up linearly from substantially zero over a period of time, say approximately one second.
The ramp is generally expressed as a variable percentage of the maximum torque that can be generated by the EPAS apparatus; this percentage is increased linearly with time. Expressing the ramp as a percentage enables the same ramp to be scaled to different apparatus having differing maximum torques and hence the ramp to be used on different vehicles.